For many years, it has been known that mounting an outboard motor to the transom of a boat spaced back from the transom a distance of several inches to more than about one foot can be advantageous for boat performance. Particularly, it has been found that the propeller of the outboard motor is given a longer moment arm for purposes of turning, thereby increasing the maneuverability of the outboard motorboat, the propeller is also given more leverage with a longer moment arm, also for purposes of trimming (i.e., angularly directing the motor up or down to keep the hull flat on the surface of the water) and the propeller blade is spaced back from the hull in water that is less turbulent due to the movement of the hull through the water. Adjustably positioning the motor up or down relative to the boat also provides proper full submersion of the propeller in the water during operation.
In the past, mounting devices have been constructed using overlappingly engaged vertical channel plates. One plate was bolted to the transom and the other plate was bolted to the motor. The plates were moveably attached to each other. Thus, the motor was attached to the boat spaced back from the transom and was adjustable up or down to accommodate different sized boats, motors and propellers. Parallel sides of the boat and mounted plate were overlappingly connected to parallel sides of the motor-mounted plate. Spaced-apart vertical slots were formed in the sides and vertically spaced pairs of bolts were extended through the vertical slots in each of the overlapping parallel sides of the boat and the motor plates. Loosening the bolts allowed the plates to be slid vertically relative to each other to position the motor up or down with its propeller at a desired distance below the hull for operating in the water. After the position was adjusted, the pairs of side bolts could be tightened to frictionally lock together the overlapping parallel sides of the mounting plates at a desired vertical position. The contact of the bolts with the inside surfaces of the vertical slots also prevented horizontal and twisting improvement of one channel plate relative to the other. The motor were thus held in place against significant torque-bending forces, shear forces and vibration, particularly when modern outboard motors with up to 200 horsepower or more are mounted spaced back from the boat using the mounting device. Because of the high forces and adverse conditions, including the vibration of the motor and the corrosiveness of the wet environment in which the mounting devices must operate, reliability of prior mounting devices has not been good. There has been a significant failure rate.
It has been found by the inventor that the failure of existing mounting devices is due to a number of conditions, including the fact that vibration loosens and allows movement of the overlapping parallel sides of the mounting devices, in spite of the intended frictional locking contact under the tension of the bolts. The result is primary load-carrying by the bolts through the vertical slots. Thus, the forces are applied through the mechanical point contact or more precisely, short line contact, of the spaced-apart bolts on the inside surfaces of the vertical slots. Such point or short line contact at the interface between the bolts and the sides of the slots results in a significant stress riser effect or focal point of the load and vibration. This, coupled with the magnitude of the forces, the significant vibration and the corrosive environment, results in high incidence of cracking failures along the vertical slots. Because the point of contact stress rising effect is so dramatic, the size or strength of the structure might need to be increased by a factor of two or three or more to reduce failures of jack plates constructed according to the prior designs. A concurrent increase in weight and cost of materials also would be required to acceptably reduce the failures.
Thus, there has been a need for a boat motor mounting device that can mount a powerful outboard motor spaced back from a boat, that can allow vertical adjustment, that is lightweight, strong, economical and that reliably resists bending and cracking.